Allow milestone snapshot saving logic to be overridden in the middleware

This change adds a flag to `SubmitRequest`: `saveMilestoneSnapshot`.
This flag defaults to `null`. If this value is left as `null`, then
milestone snapshots will be written according to the interval set in the
options passed into the provided `MilestoneDB` instance.

The value can be overridden in middleware, like in the test case in this
commit. If the value of `request.saveMilestoneSnapshot` is changed to
a `boolean`, then all interval logic is ignored, and the value of that
flag is used directly to determine if a milestone should be saved.

This allows more complex milestone logic, such as:

  - saving a milestone once per day
  - different intervals depending on collection
  - non-uniform milestone intervals

lib/submit-request.js

@@ -21,6 +21,11 @@ function SubmitRequest(backend, agent, index, id, op, options) {
   // For custom use in middleware
   this.custom = {};
 
+  // Whether or not to store a milestone snapshot. If left as null, the milestone
+  // snapshots are saved according to the interval provided to the milestone db
+  // options. If overridden to a boolean value, then that value is used instead of
+  // the interval logic.
+  this.saveMilestoneSnapshot = null;
   this.suppressPublish = backend.suppressPublish;
   this.maxRetries = backend.maxSubmitRetries;
   this.retries = 0;
@@ -220,8 +225,12 @@ SubmitRequest.prototype._addSnapshotMeta = function() {
 };
 
 SubmitRequest.prototype._shouldSaveMilestoneSnapshot = function (snapshot) {
-  return snapshot
-    && snapshot.v % this.backend.milestoneDb.interval === 0;
+  // If the flag is null, it's not been overridden by the consumer, so apply the interval
+  if (this.saveMilestoneSnapshot === null) {
+    return snapshot && snapshot.v % this.backend.milestoneDb.interval === 0;
+  }
+
+  return this.saveMilestoneSnapshot;
 };
 
 // Non-fatal client errors:

test/milestone-db.js

@@ -312,6 +312,66 @@ module.exports = function (options) {
           }
         ]);
       });
+
+      it('can have the saving logic overridden in middleware', function (done) {
+        backend.use('commit', function (request, callback) {
+          request.saveMilestoneSnapshot = request.snapshot.v >= 3;
+          callback();
+        });
+
+        db.on('save', function (saved, collection, snapshot) {
+          if (snapshot.v !== 4) return;
+
+          util.callInSeries([
+            function (next) {
+              db.getMilestoneSnapshot('books', 'catcher-in-the-rye', 1, next);
+            },
+            function (snapshot, next) {
+              expect(snapshot).to.be(undefined);
+              next();
+            },
+            function (next) {
+              db.getMilestoneSnapshot('books', 'catcher-in-the-rye', 2, next);
+            },
+            function (snapshot, next) {
+              expect(snapshot).to.be(undefined);
+              next();
+            },
+            function (next) {
+              db.getMilestoneSnapshot('books', 'catcher-in-the-rye', 3, next);
+            },
+            function (snapshot, next) {
+              expect(snapshot.v).to.be(3);
+              next();
+            },
+            function (next) {
+              db.getMilestoneSnapshot('books', 'catcher-in-the-rye', 4, next);
+            },
+            function (snapshot, next) {
+              expect(snapshot.v).to.be(4);
+              next();
+            },
+            done
+          ]);
+        });
+
+        var doc = backend.connect().get('books', 'catcher-in-the-rye');
+
+        util.callInSeries([
+          function (next) {
+            doc.create({ title: 'Catcher in the Rye' }, next);
+          },
+          function (next) {
+            doc.submitOp({ p: ['author'], oi: 'J.F.Salinger' }, next);
+          },
+          function (next) {
+            doc.submitOp({ p: ['author'], od: 'J.F.Salinger', oi: 'J.D.Salinger' }, next);
+          },
+          function (next) {
+            doc.submitOp({ p: ['author'], od: 'J.D.Salinger', oi: 'J.D. Salinger' }, next);
+          }
+        ]);
+      });
     });
   });
 };